I, like many other 6900/6910 owners (former or current) on this site, are happy with the quality of coffee produced by these machines. *Its great that youve had a long trouble-free relationship with your SB. *From the anecdotes on here and elsewhere it seems that many have not been so lucky.
Now this could just be a case of a more vocal minority i.e. youre more likely to post if youve had a bad experience rather than good. *Im very happy with the quality of coffee producd by my 6910/0480 combo and have had no machine quality issues so far, although its less than a year old and Ive only owned it for 2 months myself. *
I dont have experience with more expensive machines but would suggest the increases in quality of the end product would be minor. *The main advantages in buying a $2K+ machine are likely to be more consistency and speed when making multiple coffees (in the case of HX or dual boiler machines).
Personally I would love a Minore III but couldnt really justify spending that sort of money, particularly as I am still mastering the equipment Ive got.